There's either an error in the walkthrough or a change in the game, but giving Christina the pills at that point in the game when she goes to check her dress doesn't seem to be an option; however, you can give her the pills after she gets back from the party. Visit her at the pool at 1am and get her a glass of wine--you can add the sleeping pills then, then go to her room at 2am.
At 4pm on Tuesday you can put the sleeping pills in Elena's wine at the pool bar. You have to spy on her first by hiding in the cinema Tuesday at 21 at least once.